Influence of exercise and restricted activity on the protein composition of skeletal muscle

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Abstract
A modified method for calculating the protein composition of muscular tissue is described. An extraction procedure enables determination of the amounts of myofilamental proteins, sarcoplasmic proteins, stroma proteins, non-protein nitrogen, total nitrogen and water. A 1st series of experiments were performed on 48 guinea pigs divided into 3 groups of 16. The animals in one of these groups were kept in very small cages so as to restrict their activity, those in another group were exercised and those in the last group served as controls. The myofilamental protein content was higher in calf-muscles of the exercised animals than in either of the other 2 groups. In a 2d series of experiments rabbits were kept up to 3 years in cages designed to severely restrict their activity. This gave rise to a modification of the composition of the thigh muscles manifested by reduced myofilamental content and enhanced sarcoplasmic content. It is concluded that skeletal muscles adapt themselves to the degree of functional activity, not only by showing hypertrophy after exercise and hypotrophy after restricted activity but also by alterations in the composition of the muscle cells. Exercise thus enhances the myofilamental density in the muscle cell, whereas restricted activity reduces its myofilamental density and instead increases the sarcoplasmic content. Both these processes are liable to influence the contractile strength of the muscular tissue.